Monty Codeact

Monty CodeAct

Collapse multi-step tool chains into a single sandboxed Python execution using Pydantic Monty — a minimal, secure Python interpreter written in Rust with sub-microsecond startup.

Syntax

Tools are called as regular Python functions with keyword arguments:

content = view(path="src/main.py")
files = glob(pattern="**/*.py")
hits = grep(pattern="TODO", paths="src")
result = bash(command="git log --oneline -5")
edit(path="config.json", old_str='"debug": false', new_str='"debug": true')

Chain by sequencing:

for f in glob(pattern="**/*.py", paths="src"):
    content = view(path=f)
    if "TODO" in content:
        print(f + ": " + str(content.count("TODO")) + " TODOs")

Discover tools

python3 scripts/codeact.py --discover       # JSON manifest
python3 scripts/codeact.py --instructions   # LLM-ready reference

Tools are auto-detected based on what's installed on the host.

Execute

uv run --with pydantic-monty python3 scripts/codeact.py --auto --workspace . --code '...'

Output: {"stdout": "...", "stderr": "...", "return_value": null, "success": true}

Monty limitations

Monty runs a subset of Python. Will error on:

• Classes — no class keyword at all
• Match statements — no match/case
• f-string format specs — f"{x:<10}", f"{x:>5}", f"{x:.2f}" all fail
• str.format() — "{:<10}".format(x) fails
• str.startswith() with tuple — use or instead
• Set comprehensions — build with list + in checks
• Third-party imports — only stdlib subset
• Most stdlib — only: json, re, datetime, sys, os.environ (no os.path, no os.walk)
• Brace expansion in glob — glob(pattern="src/{db,services}/**/*.py") is supported.

Sandbox tool return types (getting these wrong causes retries):

• glob(pattern=...) → list of strings like ["src/app.py", "src/utils.py"]
• view(path=...) → string (file content)
• mcp_call(server=..., tool=..., ...) → string
• bash(command=...) → dict with keys stdout, stderr, returncode

Key usage patterns:

• Do NOT scout first. glob() and view() are in the sandbox — discover files inside your codeact program, not with separate tool calls before it.
• One program, one bash call. Do not run multiple codeact invocations. If the first one fails, fix the bug in the program, don't add a scouting step.
• Wrap file reads in try/except so one bad file doesn't abort the run.
• Use for f in glob(pattern="**/*.py"): to iterate files. No os.walk or os.path.

Output formatting workaround (use instead of format specs):

def pad(s, w):
    s = str(s)
    return s + " " * max(0, w - len(s))

Tips: Use chr(10) for newlines. Use import json explicitly. Use string concatenation (+) or simple f-strings (f"count: {n}").

Trust model

Sandboxed code can only reach the outside world through registered tool functions. Use --workspace to restrict file tools to a directory tree.

Prerequisites

• Python 3.10+
• uv (recommended) or pip install pydantic-monty

References

• references/tool-patterns.md